Illustration: Alex Nabaum

So where does ENUM fit in? Originally, it was conceived as little more than a way to use the DNS to keep track of a few phone numbers that had been moved from one carrier to another. It works, basically, by taking a telephone number, putting periods between the digits, and flipping the resulting string around so it matches the specific-to-global (or right-to-left) format of domain names. The resulting numbers go into a single global directory of telephone numbers in the DNS, located under "e164.arpa." The .arpa top-level domain has been limited to database lookups.

For example, the globally unique telephone number: +1 650 381 6115 becomes the globally unique domain name: 5.1.1.6.1.8.3.0.5.6.1.e164.arpa.

The ENUM standard specifies the use of a resource record called the Naming Authority Pointer (NAPTR) resource record, which can encode far more information than a simple 32-bit value. It does so by means of algorithmic rules which, when sent back to the requester, can be used to compute a customized answer.

ENUM is an international phenomenon. Here's an example for the U.K. phone number +44 1632960083, with data from domain name 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa:

NAPTR 10 101 "u" "E2U+email:mailto" "!^.*$! mailto:info@example.com!" .

NAPTR 10 102 "u" "E2U+sms:tel" "!^.*$!tel:+441632960083!" .

NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip@example.com!" .

The key variable here lies in the space occupied by "mailto," "tel," and "sip." These elements refer, respectively, to an e-mail application, POTS, and the SIP, the Internet Engineering Task Force's signaling protocol. (Among other things, SIP is what allows an instant-messaging buddy list to say which of your friends is online the same time you are.) So these database statements tell any inquiring network that the customer in question can be reached via SIP, via e-mail to info@example.com, or via conventional telephony. The numbers specify that the caller (or a calling software program) is asked to use SIP first, and if that doesn't work to try mail or telephony.

Multiple NAPTR records associated with a single number can provide complicated priorities and preferences for contacting users both across devices, including cellphones, PDAs, fax machines, and voice-messaging systems, and within individual ones. A cellphone alone, for example, can be used for voice, voice mail, instant messaging, text messaging, and e-mail, and each of these can have different rules for when the phone is turned on or off. One might even want to send copies of a message in several formats at once.

Of course, the very flexibility of the ENUM record makes it a tempting target for abuse, such as e-mail spam, instant-message spam, and even junk telephone calls, which will surely become common as voice telephony moves to the Internet. Quite apart from such intrusions is the threat that outsiders may try to get their hands on proprietary information, such as the contact numbers of a given company's customers or vendors.

For these reasons, communication providers may choose to put their NAPTR resource records in a secure domain outside the e164.arpa domain, that is, in a private ENUM, accessible only to privileged devices. A level of accessibility in between these two domains may also be created for carriers that wish to exchange ENUM data among themselves, to take advantage of the lowest-cost method of routing telephone calls through the Internet. We call this version Peered Infrastructure ENUM.

Public ENUM trials are under way in a number of countries, including the United Kingdom, Germany, Austria, and Australia; in some cases, these trials have to resolve political and privacy issues as well.

So, WILL the Internet handle the load? The answer is yes, with a large caveat. First of all, the "pipes" themselves—that is, the fiber-optic cables in the Internet backbone—are fat enough, and new technology is always coming along to make them effectively fatter.

But the other part of the system, the DNS, is not quite so robust as many like to think. First, a merged world will require vastly more storage than today's DNS system. NAPTR records will be larger, typically dozens of bytes, compared with today's 4 bytes for IP addresses. In addition, there will be hundreds of millions of names to track in some servers, versus today's entries, which generally run in the thousands. Even the .com database—by far the biggest—now has only 40 million names.

Second, it takes time to answer a DNS query, and such latency, as it is called, has so far been overlooked because it doesn't matter for e-mail and is generally tolerated in Web page displays. Latencies in the tens of seconds are not unusual. Telephone callers, however, expect latencies of less than half a second.

Particularly in public ENUM, subscribers will want to view and change their own contact information and will expect changes to take place immediately. Often these changes will be automated or implicit as a phone comes within range of a Wi-Fi access point, for example. Many of today's DNS servers have complex and lengthy update requirements; they will simply not meet these needs. Further, DNS servers will need to continue answering queries while they are being updated. Indeed, configuration and provisioning often require Internet service providers to take DNS servers off-line, at least briefly. People won't stand for such outages in telephone service. In short, today's DNS servers cannot handle the challenges that ENUM will pose.